Hepatitis C virus (HCV) is an enveloped, positive-sense, single-stranded RNA virus, of the genus Hepacivirus, belonging to the family Flaviviridae. Infection by HCV is a leading cause of liver disease and cirrhosis in humans. Transmission occurs primarily by way of percutaneous exposure to infected blood, typically involving use of injected drugs or injury with objects contaminated with blood, but is also associated with sexual contact with infected partners. Thanks to viral testing, risk of transmission by blood transfusion or by transplant is extremely low. Infection is often asymptomatic, or symptoms are mild, and about 15-20% of infected persons are able to clear the virus without treatment. However, infection in the remaining 80-85% of infected persons develops into persistent infection, which may be life-long, causing liver disease, which can lead to cirrhosis and hepatocellular carcinoma. HCV infection is the most common chronic blood-borne disease in the United States, affecting about 4 million people and causing about 12,000 deaths per year. “Evaluation of Acute Hepatitis C Infection Surveillance—United States, 2008,” MMWR, Nov. 5, 2010, 59(43). Approximately 170 million persons around the world have chronic hepatitis C infection. Chen et al., Int J Med Sci, 2006, 3(2):47-52. Personal consequences of HCV infection include decreased life expectancy, chronic debilitating liver disease and possibly liver cancer, and risk of infection of sexual partners and health care workers. Economic consequences of chronic HCV infection in the United States are exceedingly large. Direct medical costs have been estimated at $10.7 billion per year for the 10-year period 2010-2019, with societal costs projected to be $54.2 billion, and the cost of morbidity from disability projected to be $21.3 billion. Id.
The hepatitis C virus has been intensively studied, and much is known about its genetics and biology. For an overview of this subject, see Tan, Ed., Hepatitis C Viruses: Genomes and Molecular Biology, Horizon Bioscience, Norfolk, UK (2006). HCV has a simple genome that resides in a single open reading frame of about 9.6 kb. The genome is translated in the infected cell to yield a single polyprotein consisting of about 3000 amino acids, which is then proteolytically processed by host and viral enzymes to produce at least 10 structural and non-structural (NS) proteins. The virus is diversified in infected humans into 16 different antigenically and/or genetically identifiable subtypes or genotypes, some of which are further subdivided into subtypes.
HCV rapidly mutates as it replicates, and is believed to exist as a viral quasispecies, meaning that it mutates rapidly as it replicates to generate many competing genetic varieties of the virus having comparable evolutionary fitness. This intrinsic generation of many varieties in a single infected person makes it very difficult to isolate a single variety for development of a vaccine, and is believed to be associated with the difficulty in developing a vaccine, development of resistance of the virus to specific pharmaceuticals, and persistence of the virus in the host. It is possible that the virus able to develop into immunologically distinct quasispecies under the pressure of the immune response of the host, thereby allowing it to survive and persist.
Another factor making it difficult to develop treatments for HCV infection is the narrow range of hosts and a notoriously difficult problem of propagating the virus in cell culture. Most research has been done using pseudoparticle systems. Pseudoparticles consist primarily of nucleocapsids surrounded by a lipid envelope and contain HCV glycoprotein complexes. These pseudoparticles have been used to elucidate the early stages of the viral replication cycle and receptor binding, and to study neutralizing antibodies. Notwithstanding, pseudoparticles have a significant limitation in that they cannot recapitulate the full replication cycle. Other systems described for investigation of HCV include culture of subgenomic RNAs in Huh-7 cells, and culture in primary human hepatocytes, and surrogate models such as the bovine viral diarrhea virus (BVDV).
Significant research has also been done in synthetic RNA replicons, which self-amplify in human hepatoma cells and recapitulate much, but not all, of the HCV replication cycle. Heretofore, such replicons have been subgenomic, and have also been unable to yield infectious viral particles. Moreover, such a replicon system appears to function only using the 1b genotype of HCV (HCV1b). More recently, HCV cell culture has become possible through the isolation of the JFH-1 clone (HCV 2a). While its uniqueness remains incompletely understood, JFH-1 replicates to high levels in Huh-7 (hepatocellular carcinoma) cells and other cell types in culture, and produces infectious particles. Serial passage of JFH-1 has caused it to become genetically conditioned to cell culture conditions and it may no longer be representative of clinical isolates of the virus, but the viral particles are apparently functional virions, insofar as they are infectious in culture and in inoculated animals bearing human liver xenografts. Apparently, the efficiency of JFH-1 replication depends significantly upon the NS5B gene of the clone. Replacement with NS5B genes from other genotypes is difficult. Woerz et al., 2009, J Viral Hepat, 16(1):1-9. Other replicon systems have been developed with various replication markers and for different HCV genotypes, including HCV 1a and HCV 2a. See, Huang et al., “Hepatitis C Virus-related Assays,” Chapter 2 in Hepatitis C: Antiviral Drug Discovery and Development, S-L Tan and Y He, eds., Caister Academic Press (2011), at pp 56-57.
Approved pharmaceutical treatments include injection of interferon, typically pegylated versions including peginterferon alfa-2a (Pegasys®) or peginterferon alfa-2b (PegIntron®). Clinical use of pegylated interferon was approved by FDA in 2001. Ribavirin (e.g., Ribasphere®, Virazole®, Copegus®, Rebetol®), a guanosine analog that has broad-spectrum activity against viruses, is used to treat HCV infection, but appears not to be effective against HCV when used as a monotherapy. Current standard-of-care therapy includes administering peginterferon in combination with ribavirin. This regimen is limited because of side effects (e.g., flu-like symptoms, leukopenia, thrombocytopenia, depression, and anemia) and only moderate efficacy; success is dependent in part on the genotype predominating in the patient. See Ghany et al., Hepatology, 2011, 54(4):1433-44.
In addition to the pegylated interferon/ribavirin regimen, three different direct-acting antiviral agents have been approved for use in humans having HCV infection. These include sofosbuvir (Sovaldi®; Gilead Sciences), an NS5B polymerase inhibitor; simeprevir (Olysio®; Janssen Pharmaceuticals), an NS3 protease inhibitor and ledipasvir (Gilead Sciences), an NS5A inhibitor. Numerous alternative pharmaceutical approaches to treatment of HCV infection are still in research and development. For example, recombinant and modified interferon molecules have also been the subject of development programs, including, e.g., recombinant alfa interferon (BLX-883; Locteron®; Biolex/Octoplus) and albinterferon alfa 2b (Zalbin®; Human Genome Sciences).
The HCV protein NS3-4A, a serine protease, which is an enzyme essential for replication of the virus, has been the subject of intensive pharmaceutical research. A number of companies are seeking to develop inhibitors of this enzyme. Some of the earlier molecules are telaprevir (Incivek®, VX-950; Vertex) and boceprevir (Victrelis®, SCH503034; Merck & Co.), each of which was approved as direct-acting antiviral agent. These various molecules may be useful as single therapeutics, but some are also being investigated in combination with interferon/ribavirin therapies and/or compounds that may be effective against HCV via other mechanisms. However, viral resistance to individual protease inhibitors is believed to occur easily. Morrison and Haas, In Vivo, May 2009, 42-47.
The NS5B polymerase of HCV is also undergoing study. This protein is an RNA-dependent RNA polymerase (RdRp), which is essential for the synthesis of viral RNA, and consequently, for the completion of the viral life cycle. An overview of the NS5B protein is available at Chapter 10 of Tan, supra.
Many groups are currently working on developing inhibitors of the NS5B polymerase. Wang et al. (J Biol Chem 2003, 278(11), 9489-95) report that certain non-nucleoside molecules bind to an allosteric site on the polymerase, interfering with a conformational change required for activity. Biswal et al. (J Biol Chem, 2005, 280(18), 18202-10) report crystal structures indicating that the NS5B polymerase exhibits two conformations, with a gross structure resembling the classical fingers, palm, and thumb domains of other polymerases. This paper also show cocrystal structures for two inhibitors bound to the polymerase, and offers hypotheses on the mechanism of polymerase inhibition. Li et al. (J Med Chem, 2007, 50(17):3969-72) report on some dihydropyrone compounds that are said to be orally available allosteric inhibitors. See also Li et al., J Med Chem, 2009, 52:1255-58.
Inhibitors of NS5B may be classified broadly into three groups: nucleoside analogues (NI), non-nucleoside analogues (NNI), and pyrophosphate compounds (PPi). See, Powdrill et al., Viruses, 2010, 2:2169-95 and Appleby et al., “Viral RNA Polymerase Inhibitors,” Chapter 23 in Viral Genome Replication, Cameron et al., eds., Springer Science+Business Media 2009.
Nucleoside analogue compounds (NI), which bind at the enzyme active site and compete with natural nucleoside triphosphates, interfere with viral RNA synthesis. A number of these compounds have entered clinical trials. Nucleoside inhibitors include, for example, IDX184 (Idenix), RG7128 (RO5024048; Pharmasset/Roche), and most notably the recently-approved sofosbuvir (SOVALDI®, PSI-7977; Gilead/Pharmasset).
Non-nucleoside inhibitors, by contrast, appear to bind at allosteric sites on NS5B—of which about 4 are well characterized. Id. NNI compounds include, for example, filibuvir (Pfizer), tegobuvir (GS 9190; Gilead), VX-222 (Vertex), A-837093 (Abbott), ABT-072 (Abbott), ABT-333 (Abbott), and PF-868554 (Pfizer).
Also among the non-nucleoside inhibitors of NS5B are a series of thiophene-2-carboxylic acids and derivatives thereof. See, e.g., Chan et al., Bioorg Med Chem Lett, 2004, 14, 793-96; International patent publications WO 02/100846 A1, WO 02/100851 A2, WO 2004/052879 A2, WO 2004/052885 A1, WO 2006/072347 A2, WO 2006/119646 A1, WO 2008/017688 A1, WO 2008/043791 A2, WO 2008/058393 A1, WO 2008/059042 A1, WO 2008/125599 A1, and WO 2009/000818 A1. See also U.S. Pat. Nos. 6,881,741 B2, 7,402,608 B2, and 7,569,600 B2. See also, Yang et al., Bioorg Med Chem Lett 2010, 20, 4614-19, relating to some bioisosteres of such compounds. Other similar compounds are described, for example, in U.S. Pat. Nos. 6,887,877 B2 and 6,936,629 B2.
Pyrophosphate compounds (PPi) mimic natural pyrophosphates released during nucleotidyl transfer reactions.
Various NI and NNI compounds have shown safety or efficacy in clinical trials, but few have yet reached approval for use in treating humans. PPi compounds, by contrast, are generally in the investigational stage.
There remains a profound need for more effective pharmaceutical therapies, including medicaments that are useful as single agents or in combination with other active agents, for the treatment of hepatitis C infection in humans.